The present invention relates to a method and device for casting a strand of liquid metal which is cast into a mold and is drawn out of the mold.
In continuous casting, a strand is cast from liquid metal by means of a mold and is drawn out of the mold. An essential factor for a high quality of the strand cast in this way is keeping the casting level (i.e., the level of liquid metal in the mold) constant. It is known to regulate the casting level. In this case, it is difficult to design a controller, since the parameters of the control system, that is to say of the casting apparatus and mold, are subject to sometimes pronounced fluctuations or disturbance variables act on the casting level.
As illustrated in FIG. 1, during continuous casting, liquid metal flows via a dip pipe with an outlet orifice into the mold. The inflow of liquid metal via the dip pipe into the mold is influenced by a plug. Particularly where ferritic steels are concerned, deposits may occur on the plug or in the region of the dip pipe, with the result that the inflow is reduced. These slow disturbing influences can be compensated in a simple way by the casting level being regulated. In this case, as a result of regulation, the orifice is increased according to the extent of the deposits. If these deposits break away, however, there is an abrupt increase in the inflow of liquid metal into the mold. It has been shown that such abrupt changes in the inflow of liquid metal into the mold lead to sharp rises in the casting level and therefore to losses of quality in the cast strand.
Accordingly, the object of the present invention is to specify a method and a device for casting a strand of liquid metal by means of a mold, by means of which method and device the effects on the casting level by breakaways of deposits, for example on the plug or in the region of the dip pipe, are reduced.
The object is achieved, according to the present invention, with a method and device described herein. For example, for casting a strand, liquid metal is cast into a mold and is drawn as a strand out of the mold, the casting level, that is to say the level of the liquid metal in the mold, being regulated to a predetermined casting-level desired value using a casting-level controller. The difference between the casting-level actual value and casting-level desired value is advantageously monitored, and the casting-level controller outputs a predetermined additional value when the difference between the casting-level actual value and casting-level desired value exceeds a tolerance threshold.
The object of the present invention is additionally achieved with an exemplary embodiment of the present invention in that, for casting a strand, liquid metal is cast into a mold and is drawn as a strand out of the mold. The casting level (i.e., the level of the liquid metal in the mold) is regulated to a predetermined casting-level desired value using a casting-level controller, and the difference between the casting-level actual value and casting-level desired value advantageously is monitored. A predetermined additional value is added to the output of the casting-level controller when the difference between the casting-level actual value and casting-level desired value exceeds a tolerance threshold. The sum of the additional value and of the previous output value of the casting-level controller then forms the new output value of the casting-level controller.
According to another embodiment of the present invention, for casting a strand, liquid metal is cast into a mold and is drawn as a strand out of the mold, the casting level, that is to say the level of the liquid metal in the mold, being regulated to a predetermined casting-level desired value by means of a casting-level controller having at least one integrator, and the difference between the casting-level actual value and casting-level desired value advantageously being monitored. The output of the integrator of the casting-level controller is replaced by a predetermined additional value when the difference between the casting-level actual value and casting-level desired value exceeds a tolerance threshold. Supplementing in this sense means that the output of the integrator is replaced by the additional value or that, in a particularly advantageous way, the additional value is added to the integrator output, the sum of the additional value and of the previous output value of the integrator forming the new output value of the integrator.
In a particularly advantageous embodiment of the present invention, the tolerance threshold is adapted to the casting process, in particular as a function of the standard deviation of the difference between the casting-level actual value and casting-level desired value or of a variable equivalent to the standard deviation. It is thereby possible to prevent casting-level fluctuations due to the possible faulty detection of a breakaway, even though no breakaway has taken place.
According to yet another advantageous embodiment of the present invention, the tolerance threshold is limited to a value of between 0.02 and 0.1, of between 0.04 and 0.1 and/or of between 0.06 and 0.1. The stability of a method according to the present invention is increased in this way.